Twin genetics: Similar siblings 09/08/07. By the Wellcome Trust The strange case of semi-identical twins.

Traditionally, scientists classify twins in two main ways: identical (monozygotic) twins, who share the same genetic material, and fraternal (dizygotic) twins, who share around 50 per cent genetic similarity. However, the discovery of a pair of so-called semi-identical twins (who share around 75 per cent genetic similarity) is causing some scientists to question the current classification system.

Vivienne L Souter (Banner Good Samaritan Medical Center in Phoenix, Arizona) and colleagues examined the genetic make-up of a pair of boy-girl twins, after one twin was born with ambiguous genitalia. While one twin was anatomically male, the other was found to be a true hermaphrodite, with both ovarian and testicular tissue. Each twin contained a mixture of two types of cell, XY and XX, thought to occur when a single egg is fertilised by two sperm.

Around 1 per cent of conceptions are a result of double fertilisations, but most of these embryos do not survive. In this extremely rare case, scientists propose that a doubly fertilised egg then went on to split into two. Although this kind of twinning is thought to be extremely uncommon, it is unlikely that cases would be identified, unless - as in this case - some kind of phenotypic abnormality was detected.

Image credit: Fiona Pragoff

References

Souter VL et al. A case of true hermaphroditism reveals an unusual mechanism of twinning. Hum Genet 2007;121(2):179-85. Abstract